Abstract

Visual cryptography which consists in sharing a secret message between transparencies has been extended to color prints. In this paper, we propose a new visual cryptography scheme based on color matching. The stacked printed media reveal a uniformly colored message decoded by the human visual system. In contrast with the previous color visual cryptography schemes, the proposed one enables to share images without pixel expansion and to detect a forgery as the color of the message is kept secret. In order to correctly print the colors on the media and to increase the security of the scheme, we use spectral models developed for color reproduction describing printed colors from an optical point of view.

Figures (8)

(a) a black and white share coding a pixel of the secret message for a black and white VC scheme, (b) a colored share (cyan, yellow, magenta, black or white subpixels) coding a pixel of the secret message for a colored VC scheme and (c) a halftoned share coding a pixel of the secret message in our CM-VC scheme.

A (2,2)-CM-VCS: light magenta stacked with brown yields light brown, the target color associated to a 1-bit in the original message (case 1). The same for magenta and yellow (case 2). Inverting colors of the shadow images gives colors associated to a 0-bit, different from the target one (cases 3 and 4). These colors displayed on a calibrated screen match the ones printed on transparencies.

(a) Transmittance of a filter. (b) Transmittance through two superposed filters without taking into account the air slice between them. (c) Transmittance of two superposed transparencies separated by a thin air slice.

An example of the proposed CM-VCS for which the secret message “CM-VC” is a desatured red. There is no information about the secret message on each transparency (a) and (b). The secret content together with the color are revealed in transmission mode when the two transparencies are superposed (c) by using daylight illumination.

An example of the proposed CM-VCS for which the secret message “CM-VC” is a desatured color. There is no information about the secret content and the color on the paper (a) and on the transparency (b). The secret message is revealed when the transparency are superposed on the paper (c) and observed in reflectance mode.

Evolution of the color distance ΔE94 for target color E with respect to a transverse shift (given in fraction of the size of a halftoned share) of one of the SIs. The blue curve corresponds to measurements of the spectral transmittance of the stack. The dashed curve corresponds to a mean spectral transmittance of such a superposition computed according to Eq. (8). When the transverse shift distance is zero, the distance ΔE94 is null, and the stack color corresponds to target color E. When the transverse shift distance is 1, the color is completely different from the target color E. Below a shift dx1 < 0.1, the ΔE94 value between target color E and the stack color is less than 1, i.e. the color difference cannot be perceived. Beyond the distance dx3, the ΔE94 value is higher than 3, and a color difference is well perceptible.